In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on a hydrophilic planar surface of a substrate. When the printing plate surface is moistened with water and a lithographic printing ink is applied, hydrophilic regions retain the water and repel the lithographic printing ink, and the lithographic ink receptive image regions accept the lithographic printing ink and repel the water. The lithographic printing ink is transferred from the lithographic printing plate to the surface of a material upon which the image is to be reproduced, perhaps with the use of a blanket roller.
Imagable elements or lithographic printing plate precursors used to prepare lithographic printing plates typically comprise one or more radiation-sensitive imagable layers disposed on the hydrophilic surface of the substrate. Following imaging, either the exposed (imaged) regions or the non-exposed (non-imaged) regions of the one or more radiation-sensitive layers can be removed, revealing the hydrophilic surface of the substrate. If the exposed regions are removable, the lithographic printing plate precursor is considered positive-working. Conversely, if the non-exposed regions are removable, the lithographic printing plate precursor is considered negative-working.
Direct digital thermal imaging of lithographic printing plate precursors has become increasingly important in the printing industry in the last 30 years because of their stability to ambient light. Such precursors have been designed to be sensitive to imaging near-infrared radiation of at least 750 nm.
Negative-working lithographic printing plate precursors useful to prepare lithographic printing plates typically comprise a negative-working radiation-sensitive imagable layer disposed over the hydrophilic surface of a substrate. Radiation-sensitive photopolymerizable compositions used in negative-working lithographic printing plate precursors typically comprise free-radically polymerizable components, one or more radiation absorbers, an initiator composition, and optionally one or more polymeric binders that are different from the other noted components.
In recent years, there has been an emphasis in the industry for simplification of the lithographic printing plate making process, including an omission of the pre-development heating step (preheat) and carrying out development on-press (DOP) using a lithographic printing ink, fountain solution, or both, to remove unwanted (non-exposed) imagable layer materials on the lithographic printing plate precursors. Such negative-working lithographic printing plate precursors must be designed by balancing many features within the element structure in order to achieve optimal press life, on-press developability, and scratch resistance. It has not been an easy task to achieve high quality in all of these properties because what chemical composition or structural features may provide optimal level in one or two properties may cause a loss in another property.
Independently of the type of lithographic printing plate precursor, lithography has generally been carried out using a metal-containing substrate comprising aluminum or an aluminum-alloy of various metal compositions, for example containing up to 10 weight % of one or more of other metals known in the art for this purpose. The raw stock aluminum-containing material can be cleaned in a “pre-etch” process using a base or surfactant solution to remove oil, grease, and other contaminants on the planar surface of the raw stock aluminum-containing material. The cleaned planar surface is then generally roughed by electrochemical or mechanical graining, followed by a “post-etch” treatment to remove any contaminants (“smut”) formed during the graining process. Further industrial details of the preparation of useful substrates for lithographic printing plate precursors are found in U.S. Patent Application Publication 2014/0047993 A1 (Hauck et al.).
After further rinsing, the planar surface of the aluminum-containing substrate is then anodized one or more times to provide an outermost hydrophilic aluminum oxide layer for abrasion resistance and other properties of the resulting lithographic printing plate precursor once one or more imagable layers have been formed thereon.
One or more anodizing processes are used in some known methods of making precursor substrates, for example, as described in U.S. Pat. No. 4,566,952 (Sprintschnik et al.) and U.S. Pat. No. 8,783,179 (Kurokawa et al.), U.S. Patent Application Publications 2011/0265673 (Tagawa et al.), 2012/0192742 (Kurokawa et al.), 2014/0326151 (Namba et al.), and 2015/0135979 (Tagawa et al.), and EP 2,353,882A1 (Tagawa et al.).
In these known methods of making precursor substrates, sulfuric acid, phosphoric acid, or both sulfuric acid and phosphoric acid have been used as electrolytes in combination with various process parameters in order to produce one or more anodic (aluminum oxide) layers of specific structures and thus achieve specific properties in the resulting precursors. However, it has been found that lithographic printing plate precursors prepared according to these known methods are still unsatisfactory in one or more precursor properties such as scratch resistance, on-press developability, and press life.
Thus, there remains a need to balance the manufacturing conditions, especially during anodization, for negative-working lithographic printing plate precursors so that improved scratch resistance is achieved without sacrificing press life and on-press developability.